• WHY WE EXPLORE
• PLANETARY NEWS
  • Archive
  • Our RSS News Feeds
  • Asteroids and Comets
  • Extrasolar Planets
  • Our Solar System
  • Search for Life
  • Solar Sailing- LightSail
  • Space Missions
  • Space Telescopes
  • The Planetary Society
• SPACE TOPICS
• FOR KIDS

browse projects: Apophis Mission Design Competition Asteroids - The Potential Threat Carl Sagan Fund for the Future Catalog of Exoplanets FINDS Exo-Earths International Year of Astronomy LIFE Experiment: Phobos LightSail - Solar Sailing Mars Climate Sounder Team Website Messages from Earth Microrovers Mirror Bees: Planetary Defense Observing Earth Pioneer Anomaly Planetary Microphones SETI Optical Telescope SETI Radio Searches SETI@home Shoemaker NEO Grants Space Advocacy Space Information Stardust@home

browse space topics: 2001 Mars Odyssey Akatsuki (Venus Climate Orbiter / PLANET-C) Asteroids and Comets Cassini-Huygens Chandra X-Ray Observatory Chandrayaan-1 Chang'e 1 Compare the Planets Curiosity (Mars Science Laboratory) Dawn Deep Impact Earth Extrasolar Planets Genesis Hayabusa (MUSES-C) Hubble Space Telescope IKAROS Jupiter Kaguya (SELENE) Lunar Reconnaissance Orbiter (LRO) Mars Mars Exploration Rovers Mars Express Mars Global Surveyor Mars Reconnaissance Orbiter Mercury MESSENGER Moon Near Earth Objects Neptune New Horizons Past Missions Phoenix Planetary Analogs Planetary Exploration Timelines Pluto and Charon Private Missions Rosetta Saturn Search for Extraterrestrial Intelligence SMART-1 SOHO Space Imaging Spitzer Space Telescope Stardust Trans-Neptunian Objects Ulysses Uranus Venus Venus Express Voyager Weekly Planetary Radio Trivia Contest WISE

Planetary News: Asteroids and Comets (2005)

First Triple Asteroid Discovered

Only weeks after scientists found a possible 10th planet in the Solar System, a different group of astronomers announced the discovery of another remarkable object: a mere asteroid with a tiny “solar system” all its own. 87 Sylvia, one of the largest asteroids orbiting the Sun between Mars and Jupiter, appears to have company: two moons, only a few kilometers across, which orbit it regularly along its equatorial plane. This makes 87 Sylvia the first “triple” asteroid ever discovered.

The discovery was announced in the August 11 issue of Nature by astronomers Franck Marchis of U.C. Berkeley, and Pascal Deschamp, Daniel Hestroffer, and Jerome Berthier of the Observatoire de Paris. “People have been looking for multiple asteroid systems for a long time” said Marchis; “I can’t believe we found one.”

Marchis and his colleagues made their discovery with the European Southern Observatory’s 8 meter telescope (Yepun) of the Very Large Telescope array at Cerro Paranal in Chile, using the telescope’s Adaptive Optics system to compensate for atmospheric interference. The images of the asteroid were recorded on the telescope’s highly sensitive infrared digital camera.

While this week’s announcement marks the discovery of the first asteroid known to possess two moons, asteroids with one moon have become quite plentiful in recent years. 87 Sylvia itself was known to have one companion ever since the discovery of its first moon in 2001. This made it one of around known 60 “double asteroids” spread among the different asteroid population of the Solar System. The realization that the asteroid had another moon came in March 2005, when Marchis sent his Parisian colleague Deschamp data collected by the VLT the previous year. “87 Sylvia est Triple?” came Deschamps’ quick response. And so it was – on several of the VLT’s images, faint but undoubtedly there, was 87 Sylvia’s second moon.

87 Sylvia is named after Rhea Sylvia of Roman mythology, so it is only natural that her two moons are named after Sylvia’s two sons – Remus and Romulus, the founders of Rome. Despite their impressive names, however, the twins are tiny: Romulus is about 18 kilometers (11 miles) across, and Remus is even smaller at 7 kilometers (4.5 miles) from end to end. Romulus, discovered (though not named) in 2001, orbits the main asteroid every 87.6 hours at an average distance of 1360 kilometers (850 miles); Remus, whose discovery made 87 Sylvia the first triple asteroid, completes an orbit every 33 hours at an average distance of 710 kilometers (450 miles). Both moons circle the asteroid in a “prograde" direction, i.e. the same direction as the asteroid moves around the Sun. Their orbits are near-circular and are close to 87 Sylvia’s equatorial plane.

All this provides scientists with a great deal of information about 87 Sylvia and its companions. Measured from Earth, the main asteroid is around 280 kilometers (175 miles) in diameter. Since the moons’ orbits determine the precise mass of the asteroid, Marchis and his friends could now calculate its density. This came out to be only 1.2 grams per cubic centimeter, which suggests to astronomer that 87 Sylvia is most likely a “rubble pile” asteroid – a loose and disordered aggregate of ice and rubble. “It could be as much as 60% empty space,” said Daniel Hestroffer, a member of the discovery team.

Knowing the structure of the asteroid could also help astronomers determine the history of the first triple asteroid. According to Marchis and his colleagues, 87 Sylvia was probably formed when an ancient asteroid collided with another and crumbled under the impact. Over time the debris from the impact re-accreted to form the loose rubble pile of the current asteroid. The two moons, Remus and Romulus, are most likely pieces of debris left over from the ancient collision, which were later recaptured by the newly formed asteroid. The fact that the moons move in a circular prograde orbit, close to the asteroid’s equatorial plane, also lends support to this scenario.